This invention relates to a hydraulic brake system including a master cylinder, wheel cylinders, a wheel slip control (antiskid) apparatus, a valve device controlling a fluid flow from a fluid source to the pressure chamber of the master cylinder, and valves actuated by the wheel slip control apparatus to control, in the event of an imminent locked condition of a wheel, the pressure in the associated wheel cylinder independently of the pressure in the pressure chamber.
A device of this type is known from German Pat. DE-OS 2,443,545. The piston of a master cylinder isolates the master cylinder pressure chamber from a fluid chamber receiving fluid under pressure from the fluid flow of the hydraulic brake booster. Axial bores in the master cylinder piston connect the fluid chamber to the pressure chamber, the bores are, however, covered by the cup seal of the master cylinder piston in the pressure chamber. In this manner, the effect of a check valve is obtained permitting exclusively a fluid flow from the fluid chamber to the master cylinder pressure chamber while inhibiting flow in the opposite direction. During a normal brake application, the pressure building up in the pressure chamber will be the same as in the fluid chamber so that fluid flow does not take place through the axial bores. However, if the wheel slip control apparatus intervenes, lowering the pressure in a wheel cylinder and connecting the wheel cylinder again to the master cylinder after the pressure decrease has taken place, the pressure in the pressure chamber will be slightly lower than in the fluid chamber for a short time after connection to the pressure chamber. Fluid will be supplied through the axial bores immediately so that the fluid volume in the pressure chamber cannot become exhausted.
A brake system of this type, however, subjects the cup seal of the master cylinder piston to excessive loads. Moreover, it is not possible to determine cup seal leaks without removing the cup seal.
From German Pat. DE-OS 2,745,354 a brake system is known in which from a specific amount of piston stroke on the pressure chamber of the master cylinder is connected to the fluid flow of the hydraulic brake booster. This stroke-dependent control is, however, very expensive and necessitates likewise a dual action of the master cylinder cup seals so that these are again subjected to an enormous load.